1. Technical Field
The present invention relates to a process for production of electrodes for electric double layer capacitors which are suitable for use for producing an electric double layer capacitor having large capacity and high power.
2. Background Art
An electric double layer capacitor has characteristics such as long service life, high cycle characteristics, and characteristics of charge and discharge with heavy current since there are no chemical reactions during charge and discharge of the capacitor as there are in a conventional secondary battery. Therefore, this capacitor is attracting much attention as a new type of storage battery or as a driving power supply for automobiles and devices. In particular, an electric double layer capacitor having large capacity and high power is being developed.
As a process for producing such an electric double layer capacitor, a method in which conductive adhesive is coated on a collector sheet such as aluminum foil and an electrode forming sheet is joined through this conductive adhesive is known (see Japanese Unexamined Patent Application Publication No. 11-162787). In this method, activated carbon, conductive carbon, binder, and solvent are mixed and kneaded first, and the electrode forming sheet is obtained by rolling and drying. A conductive filler such as carbon black or graphite and a binder comprising a resin component such as polyvinyl alcohol (PVA) are mixed to prepare a conductive adhesive, and then the conductive adhesive is coated on the collector sheet and joined with the electrode forming sheet and is dried.
However, if the electrode forming sheet is completely dried during the producing process of the electrode forming sheet, large amounts of conductive adhesive which is coated to the collector sheet is drawn into gaps of the electrode forming sheet. Therefore, the amount of the conductive adhesive must be increased to maintain joining strength. Furthermore, if the conductive adhesive is absorbed into gaps of the electrode forming sheet, electrostatic capacity may be decreased and the weight may be increased. In particular, when the capacitor is used for automobiles, plural cells are connected in series as the driving power supply, and slight increases in weight or slight decreases in electrostatic capacity of a single cell can cause serious problems in the overall driving power supply. Furthermore, in this case, the capacitor is used under severe conditions, the temperature of the electrolyte may be increased and interface separation may easily occur. Therefore, adhesion strength must be increased.